The Ab- Or System
This system is important in the study of granitic rocks. With the addition of Qtz to the Ab-Or system, it is a first approximation of a true granite.
The appearance or topology of the Ab-Or system changes with pressure. At pressures < 4 kbars the system displays complete solid solution between Ab and Or. At pressures > 4 kbars the system displays partial solid solution.
Another important aspect of the system is the presence of the solvus, below which the feldspar solid solution undergoes unmixing to produce two separate solid feldspars, one rich in the Ab component and one rich in the Or component.
Overall the effects of increased pressure on this system are:
- Disappearance of leucite from the binary diagram.
Leucite is not stable at P > 2 kbars. Leucite is not found in plutonic rocks which have crystallized at depths corresponding to pressures in excess of 2 kbars. Leucite is most commonly found in volcanic rocks, which are undersaturated with respect to silica.
- Increasing P also lowers the melting point of both end member components, Ab and Or.
- Nature of the phase diagram changes - the minimum disappears, it is replaced by a eutectic, and the solid phases exhibit partial solid solution.
For the 5 kbar diagram of this system, the results of experimental work are shown. This system has been determined using 20 mixtures (isopleths) of Ab and Or which were equilibrated at a variety of temperatures, then quenched followed by examination of the phases present. The experimental results are recorded on the diagram using the following symbols:
- red circles for liquid,
- green circles for liquid and solid, and
- blue circles for solid.
The isopleths are represented by the vertical columns of symbols. The gas phase is water vapour, which is present in excess, as water is used as the pressure medium.
Look at the 2 kbar diagram in detail.
Crystallization and melting paths in this system are comparable to those examined in the Ab-An system. As liquids cool between the liquidus and solidus the solid phase is constantly changing compostion, along the solidus, to maintian equilibrium with the liquid, which also is changing compostion, along the liquidus.
The aspect of this diagram which is different from previous diagrams examined is the presence of the solvus, below which the feldspar solid solution unmixes to two separate feldspars, one qich in Ab and one rich in Or.
This exsolution results in the formation of a variety of textures which can be recognized by examining the resulting rock under the microscope.
- If the feldspar grain consists of an Or-rich host and exsolution lamellae of Ab, then the grain is a perthite and exhibits a perthitic texture.
- If the feldspar grain consists of an Ab-rich host and lamellae of Or, the grain is an antiperthite, resulting in an antiperthitic texture.
